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Venus and Serena aren’t the only ones holding court this summer. Suddenly, sisters seem to be everywhere: Kate and Pippa; Kim, Kourtney and Khloe; Jessica and Ashlee. And while some are as likely to squabble as share, this latest crop of sisters is better known for having each other’s backs than for family feuds.

It’s no surprise to researchers in the U.K., who found that having a sister is likely to make you better emotionally balanced than those who grew up with a brother or as an only child. “People with sisters tend to have better mental and physical health as they get older,” says Katherine Conger, associate professor of human development and family studies at the University of California-Davis. “Sisters are the communicators in the family.”

On the other hand, when sisters feel their relationship is strained, their happiness takes a hit in a way that doesn’t happen with brothers. “I found that when adult sisters were estranged, they usually expressed deep regret and feelings of loss,” says Marcia Millman, professor of sociology at the University of California-Santa Cruz and author of The Perfect Sister.

One reason that a tight bond may matter more to sisters: They use each other as a reality check.

“We share just about everything,” says Sophie Kallinis LaMontagne, 34, who spends “nearly 18 hours a day” with her sister, Katherine Kallinis Berman, 33 (at left). Though they were always close, it wasn’t until they started a business together that they became constant companions.

As co-owners of Georgetown Cupcake, an expanding empire of cupcakeries, the siblings work round-the-clock. “Having your sister as your business partner gives you a huge head start,” LaMontagne says. “I’m not saying it’s easy, but if anything goes wrong, you know you’ve got your sister behind you.”

Sweet dreams

Best of friends growing up, Berman and LaMontagne went their separate ways after college. LaMontagne became a biotech expert; Berman headed into the world of fashion. But a frank conversation in 2007 brought the two back together again.

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“Ever since we were kids, we’d talked about opening a bakery together,” LaMontagne says. “And suddenly we were just like, ‘If not now, when?’ So we quit our jobs, borrowed our grandmother’s cake recipes and took a leap of faith.”

Five years later, Georgetown Cupcake has gone from a two-person shop in Washington, D.C., to a 350-employee enterprise with stores on both coasts, not to mention a reality series on TLC.

Sisterhood is a component of the company’s success. “There are no boundaries to what we can tell each other,” Berman says. “We are candid and honest about what we think works or doesn’t work.” Their arguments can be heated, but the sibs say it has made their business better. “There is no ego involved,” LaMontagne says. “We take the best of each other’s ideas.”

Sibling competitiveness can be an advantage, says Jeffrey Kluger, author of The Sibling Effect. “As siblings, we’re hardwired to compete for resources — mom’s affection, good grades, who is the better athlete in school,” he says. “Competition can be healthy. It forces you to be even better at something than you were.”

Sociologist Congers agrees. “Would either Williams sister be as good at tennis on her own? I don’t think so. Having a sister as a daily training partner elevates each player’s game.”

Sister shorthand

Some of the most successful collaborations happen when sisters merge talent. Marty Munson, 48, a magazine and digital media consultant in New York City, frequently combines brain cells with her sister Mimi Kolb, 51, an art director and creator of Bento Box Gifts, in Reading, Pa.

“A brainstorm with the two of us can yield in a few minutes what it might take an hour or more to achieve with people who don’t know each other so well,” Munson acknowledges.

She recalls the time she mentioned to Kolb that she needed new business cards. “She whipped out her computer and designed them on the spot, automatically knowing the look I was going for,” Munson says. Thanks to sister shorthand like “That pink looks too much like your junior prom skirt,” the project captured Munson’s style and skill set perfectly.

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Even when sisters follow divergent paths, their success can be traced back to their connection. “You’ll see situations where growing up, the older sister is great in school, getting top grades, and the younger sister thinks, ‘I’m never going to be able to beat those marks, but maybe I can be equally successful in something else,’” says Laurie Kramer, professor of applied family studies at the University of Illinois.

And what about that stereotype of out-of-control sister rivalries? “There is always great interest in a ‘catfight’ between female siblings, while the more common experience of supporting each other is downplayed,” Millman says, adding that sibling relationships are complex.

Of course, if you have a sister, you probably already know that feeling. If your relationship falls more on the competitive end of the spectrum, just wait: Research shows sisters tend to get closer as they get older. After all, Kluger points out, “your sister is likely to be the longest relationship of your life.” Better make it a good one.

Aug. 5 is National Sisters Day. To celebrate, the Georgetown Cupcake sisters will combine their favorite flavors to create Chocolate Hazelnut Caramel, online and in stores.